The present invention pertains to a magenta ink for inkjet printing and in particular to a magenta ink having a mixture of specific magenta colorants including a self-dispersed magenta pigment and a xanthene dye. The self-dispersed magenta pigment is defined as a magenta pigment surface modified with bisphosphonate functional group or sulfonate functional group.
Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink may be driven toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven from a nozzle toward a medium by an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezoelectric element in the ink nozzle. Electrically-caused distortions of the piezoelectric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from the nozzle toward the print medium by the formation of an expanding vapor phase bubble in the nozzle. These various printing methods are described in “Output Hard Copy Devices,” edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13, entitled “Ink Jet Printing”).
Ink jet printers are well known. One common type of ink jet printer uses a replaceable print cartridge having a printhead and a supply of ink contained within the cartridge. The printhead is installed in a printhead carrier, which positions the printhead along a printing zone. When the supply of ink contained within the print cartridge is depleted, the print cartridge is disposed of and a new print cartridge is installed in the printhead carrier. In contrast, off-carrier inkjet printers deliver ink through supply tubes connected from a replaceable off-carrier ink supply tank to an ink jet printhead positioned on the printhead carrier. This inkjet printhead is not disposable but permanent or semi-permanent in nature. Naturally consumers expect that these permanent or semi-permanent printheads have a longer life compared to a disposable printhead. When the supply of ink is exhausted, the consumer will purchase a new tank filled with ink as opposed to purchasing a brand new printhead containing the same supply of ink. Purchasing a tank of ink is a more economical option for the consumer. Therefore it is imperative that a permanent or semi-permanent printhead does not fail in their operations prematurely because consumers expect that permanent and semi-permanent printheads will have a longer life compared to a disposable printhead.
Ink being jetted over the life of permanent or semi-permanent printheads can cause many problems which affect the overall performance of the printhead. One of the most common problems is kogation. During the millions of firing of ink drops from the printhead, the layer of ink covering the surface of the heating element of the printhead can reach a very high temperature, usually over 300° C. At this high temperature, ink can decompose, thereby depositing a residue onto the surface of the heater. This phenomenon is called kogation. The presence of this residue negatively affects the volume, mass, shape and velocity of each ejected drop of ink jetted from the printhead, thereby reducing the quality and the expected life of a thermal inkjet printhead. A loss of drop mass over the life of the printhead negatively reduces the accuracy of drop placement onto the print media. In extreme cases, kogation causes the printhead to stop working altogether. Therefore, it is necessary to have an ink that does not cause the undesirable kogation in a printhead.
Another important factor considered when formulating inks is reduced idle time. Idle time is used to measure the short term reliability of an ink. Idle time is measured as the time between nozzle firings just before the printhead produces delayed or misdirected ink droplets. It affects the maintenance algorithm of the printer which in turn affects the through-put and how much ink will be used during the maintenance of the printhead. Often magenta inks exhibit poor idle time.
Post print paper curl is also an issue, especially during fast speed printing. The use of water in high concentrations in inkjet ink formulations induces the water to negatively react with the paper, thereby causing the edges of the paper to migrate towards the center of the paper. Curl may appear immediately after printing or it may take a couple of days to manifest itself. In its final state, the paper may curl so much that it resembles a roll, scroll or a tube. Curled paper cannot be stacked nor can it successfully be duplexed in a print job.
In order to reduce paper curl it is helpful to understand the mechanism of paper curl and determine which particular ink components have an effect on this paper curl. Media tends to curl after a large quantity of ink is deposited onto the surface of the printing substrate. Plain paper substrates are comprised mainly of cellulose fibers, along with varying levels of inorganic fillers. It is the interaction of the water in the inkjet inks with these cellulose fibers that leads to the phenomenon of paper curl. The absorption of water by the cellulose fibers causes swelling and then breaking of the interfiber cellulose bonds in the paper.
Upon drying there are differential stresses between the printed and non-printed surfaces. These differential stresses manifest themselves as paper curl, whereby the substrate tends to curl towards the surface from which moisture was last removed (the imaged surface). An ink formulation with a reduced level of water in addition to humectants with high boiling points effectively eliminates the typical end user problems of stacking and displaying printed images with acceptable levels of paper curl.
Ink formulations used in ink jet printers comprise either a soluble dye or an insoluble pigment. Unfortunately, inks comprising soluble dyes can exhibit many problems, such as poor water-fastness, poor light-fastness, clogging of the jetting channels as a result of solvent evaporation and changes in the dye's solubility, dye crystallization, poor print quality including ink bleeding and feathering, poor thermal stability, chemical instability, and ease of oxidation.
Pigmented inks are also not problem free. For example, insoluble pigments must be present in the ink as a dispersion. Unfortunately, traditional polymer dispersed pigmented ink is not vibrant due to its penetration into fibers on plain and Colorlok® papers. Another problem with pigmented ink is the propensity of the ink particles to settle during storage. This can lead to clogged nozzles and poor print quality.
As discussed above, it has been very difficult to develop a magenta ink formulation which optimizes all of these desired ink printing properties simultaneously. Therefore, many trade-offs arise when trying to formulate an acceptable magenta ink formulation. Often the inclusion of an ink component meant to fix and or control one of the above discussed problems can prevent another printing property from being met.
Prior to the present invention, however, an ink formulation which optimizes all of these desired ink printing properties had not been achieved. For example, increasing the pigment load in the inkjet ink formulation improves the optical density and gamut of the ink but it also has a negative impact on jetting and heater kogation. Many solvents help kogation but they negatively increase the viscosity of the ink. A certain viscosity value is vital, especially when the ink is used in an off-carrier printer. Low viscosity is desirable for fluids used in off-carrier printing systems. Moreover, low viscosity also increases the ink penetration and image drying speeds. Usually a desirably magenta ink viscosity at 25 C.° is in the range of 2.5-3.0 cps. However, many solvents and antikogation agents negatively increase the viscosity of the ink. This causes great difficulty in jetting the ink, especially after the printhead is idle, and consequently leads to clogging of the printhead, difficulty in jetting the ink and ultimately to the printhead failing prematurely.
Humectants (also termed cosolvents) can be added to the ink composition to aid in maintaining the colorant in the ink composition and to enhance the performance of the ink. However, often the addition of particular humectants can negatively impact the print quality of the ink. Unfortunately, high quantities of humectants adversely affect the magenta ink in terms of viscosity, dry time and smudging. Consequently, there is a need to balance these competing factors when deciding exactly which components to include and at what percentage each component should be used in a magenta ink formulation, wherein the ink formulation would minimize kogation and paper curl and improve idle time while still having acceptable print quality and print properties. The magenta inkjet ink of the present invention balances these many trade-offs to formulate an optimized magenta inkjet ink formulation.
The magenta ink of the present invention uses a particular combination of magenta colorants which surprisingly produces an optimal ink formulation which minimizes kogation and paper curl and improve idle time while still having acceptable print quality and viscosity. With the increased usage of off carrier inkjet printing systems having permanent and semi-permanent printheads, this type of magenta inkjet ink formulation is greatly needed.
It has now been discovered that the use of a very specific mixture of magenta colorants including a pigment surface modified with a particular functional group and a xanthene magenta dye provides a unique blend of optimized properties needed and desired for a magenta pigmented ink. This magenta ink composition provides excellent printing properties in terms of stability, optical density (even when low levels of pigment are utilized), viscosity, paper curl, printing characteristics (water-fastness, minimized feathering, minimized running of ink on the printed page), and printer maintenance problems (i.e., minimized clogging of the printhead during gaps in printer usage).
It is, therefore, an object of the present invention to provide an improved magenta pigmented ink composition for ink jet printers having optimal chroma, gamut value and viscosity while simultaneously reducing paper curl and kogation and improving idle time. The magenta inkjet ink of the present invention is especially suitable for use in permanent or semi permanent printheads. Other objects and advantages of the present invention will become apparent from the following disclosure.